Holographic memory systems are known as memory systems in which information is recorded or reproduced optically to and from a holographic recording medium consisting of a photopolymer or the like.
In a holographic memory system, when data is recorded onto a holographic recording medium (hereinafter, simply called “recording medium”), two-dimensional demodulation is carried out on the basis of the input data to form units of two-dimensional data known as data pages, which are displayed on a spatial light modulator in which a plurality of pixels for modulation are arranged in a two-dimensional configuration, and the light is thereby modulated spatially to generate signal light. The signal light and a reference light beam interfere with each other on a recording medium and the corresponding interference pattern is recorded onto the recording medium.
On the other hand, when two-dimensional data is reproduced from the recording medium, a reproduction light beam (diffracted light) is generated by irradiating only the reference light beam onto the recording portion of the recording medium under the same conditions as those used during recording, and a reproduction image created by the reproduction light is received by an image sensor and the original data page is thereby reproduced.
There are cases where the original input data cannot be reproduced accurately due to deformation of the output signal as a result of the effects of noise in the holographic memory system, and therefore in order to prevent this, a method which uses a so-called decision feedback Viterbi detection process has been proposed (see Patent Document 1).
This decision feedback Viterbi detection process utilizes the fact that the data being handled is two-dimensional data and assumes that in a matrix type of image sensor comprising a plurality of photoreceptor pixels arranged in a square configuration, for example, the reproduction process has been performed accurately in the row immediately above the current row (of photoreceptor pixels) and therefore carries out Viterbi detection on the basis of the output signal from the current row while subtracting the effects of the row immediately above. In other words, decision feedback is performed in the column direction and the pattern of change in the column direction is also employed in the Viterbi detection process. For example, when reproducing the input data on the basis of the output signal received in the uppermost row, it is known that the amount of received light in the row (virtual row) further above this uppermost row is zero, and therefore, when reproducing the input data on the basis of the output signal from this uppermost row, the data is reproduced directly without alteration and no decision feedback is applied. Furthermore, when reproducing the input data on the basis of the output signal from the second row from the top, then it is assumed that in the uppermost row the input data has been reproduced on the basis of the output signal without being affected by the row further above the uppermost row, and hence the data is reproduced by subtracting the effects of the input data of the uppermost row. Moreover, when reproducing the input data on the basis of the output signal from the third row from the top, it is assumed that in the second row from the top the input data has been reproduced accurately on the basis of the output signal, and hence the data is reproduced by subtracting the effects of the input data of the second row from the top. In this case, the direction of the Viterbi detection process is the row direction from left to right, for example, in the image sensor, and the direction of the decision feedback is the column direction.
However, in the conventional decision feedback Viterbi detection process described in Patent Document 1, there is a problem in that decision feedback Viterbi detection cannot be carried out accurately from the reproduction image on the image sensor, due to the fact that the positional relationships between the respective pixels in the spatial light modulator and the corresponding pixels of the image sensor actually deviate in various directions (namely, so-called “pixel deviation” occurs). In other words, if there is a difference between the direction of the pixel deviation and the direction of decision feedback, then it is not possible to carry out decision feedback Viterbi detection.
A method has been proposed according to which, when reproducing a data page on a recording medium by using a decision feedback and Viterbi reproduction process, the direction of relative pixel deviation between one pixel in the spatial light modulator and one photoreceptor pixel in the image sensor corresponding to same is determined from the transfer function of the whole optical system, and the decision feedback direction and the state block shape in the decision feedback Viterbi reproduction processing are set on the basis of the direction of pixel deviation determined as described above (see Patent Document 2).
Viterbi detection is described in paragraphs (0120) to (0120) of Patent Document 2 and the corresponding trellis diagram, but a branch metric BkBk=(rk−sk)2  [Expression 1](where sk is the ideal output of the state transition at time k) is calculated as the output result of the Viterbi detection process, and a path metric Lk
                                          L            K                    ⁢                                    ∑                              k                =                1                            K                        ⁢                          B              k                                      =                                                            ∑                                  k                  =                  1                                                  K                  -                  1                                            ⁢                              B                K                                      +                          B              K                                =                                    L                              K                -                1                                      +                          B              K                                                          [                  Expression          ⁢                                          ⁢          2                ]            is calculated in respect of two paths leading to the respective states Sn (n=1, . . . , 4) at time k, and the path having the smallest path metric Lk is left as the survivor path leading to the respective states.
A metrics calculation is carried out for each pixel (bit) unit, the smallest path metric is selected from all of the combinations, and the state value which constitutes this survivor path becomes the determination result for this row.
On the other hand, in a holographic memory system, a non-uniform light intensity distribution is generated in the reproduction image, due to the intensity distribution of the light source, the optical components used, and the non-uniformity of the properties of the recording medium, and other factors. Therefore, in order to prevent increasing error when judging the pixels in the data page, a two-dimensional modulation process, such as a 1:2 differential code, 2:4 differential code, or 4:8 or 6:8 balanced code, or the like is used (see Patent Document 3). Two-dimensional modulation is two-dimensional encoding which generates a bit pattern in which information bits of two types, namely, ON (bright) and OFF (dark), are arranged in a two-dimensional configuration; each of the data bits which are to be recorded is converted into a unit symbol (a two-dimensional modulation pattern symbol) consisting of the prescribed number of pixels in the spatial light modulator, and hence the input data is thereby converted into a set of a plurality of unit symbols.    Patent Document 1: U.S. Pat. No. 5,740,184    Patent Document 2: Japanese Patent Kokai No. 11-317084    Patent Document 3: Japanese Patent Kokai No. 2001-75463
If a data page is modulated two-dimensionally, then in conventional decision feedback Viterbi detection, since the detection processing advances one pixel at a time, there has been a possibility that the detection result will not match the unit symbols of the two-dimensional modulation rules (the two-dimensional modulation pattern symbols).
For example, in the case of 2:4 modulation, a modulation rule applies according to which only one pixel in a unit symbol consisting of four pixels is white and the other three pixels are black, but there are cases where the symbol resulting from decision feedback Viterbi detection contains two white pixels.
Moreover, since the result of decision feedback Viterbi detection is in a code based on the two-dimensional modulation pattern symbols, then a demodulation process for reverting to the original data is required.
Furthermore, conventionally, pixel deviation has been detected in advance, before carrying out data reproduction by the decision feedback Viterbi detection process. For example, it is detected just once when the power supply is switched or when the recording medium is changed, for instance. However, in practice, when a hologram is being reproduced, movement of the recording medium or variation in the angle of the reference light, or other such factors cause movement in the reference position of the reproduction light (reproduction image) on the image sensor, and consequently, there is variation in the optical transmission factor. In cases such as these, there is also a problem in that the decision feedback Viterbi determination does not work correctly and the error augments.
Therefore, one example of the problem to be solved by the present invention is to provide a two-dimensional demodulation method and a holographic apparatus whereby error can be reduced.
Measure Taken to Solve the Problem
The two-dimensional demodulation method according to the present invention is a two-dimensional demodulation method for reproducing a data page, by means of maximum likelihood decoding processing, from a recording medium on which the data page has been recorded as a set of a plurality of two-dimensional modulation pattern symbols, each of which comprises a plurality of pixels, and which have been modulated two-dimensionally, the method comprising: a step of determining a value of pixel deviation in a reproduction image obtained by receiving light from the recording medium on an image sensor; a step of splitting an output signal of the image sensor, which indicates the reproduction image of the light received from the recording medium, into signal symbol data by division into blocks in accordance with the two-dimensional modulation pattern symbols; and a step of carrying out maximum likelihood decoding for each of the signal symbol data in accordance with the pixel deviation value.
The two-dimensional demodulation apparatus according to the present invention is a two-dimensional demodulation apparatus which reproduces a data page by means of maximum likelihood decoding processing from a recording medium on which the data page has been recorded as a set of a plurality of two-dimensional modulation pattern symbols, each of which comprises a plurality of pixels, and which have been modulated two-dimensionally, comprising: an image sensor; means for determining a pixel deviation value for a reproduction image obtained by receiving light from the recording medium on the image sensor; means for splitting an output signal of the image sensor which indicates the reproduction image of the light received from the recording medium into signal symbol data by division into blocks in accordance with the two-dimensional modulation pattern symbols; and means for carrying out maximum likelihood decoding for each of the signal symbol data in accordance with the pixel deviation value.
The holographic apparatus according to the present invention is a holographic apparatus which reproduces a data page by means of maximum likelihood decoding processing from a recording medium on which the data page has been recorded as a set of a plurality of two-dimensional modulation pattern symbols, each of which comprises a plurality of pixels, and which have been modulated two-dimensionally, comprising: an image sensor; means for determining a pixel deviation value for a reproduction image obtained by receiving light from the recording medium on the image sensor; means for splitting an output signal of the image sensor which indicates the reproduction image of the light received from the recording medium into signal symbol data by division into blocks in accordance with the two-dimensional modulation pattern symbols; and means for carrying out maximum likelihood decoding for each of the signal symbol data in accordance with the pixel deviation value.
Maximum likelihood decoding is a method in which the decoded signal does not provide a direct representation of the original bit sequence but rather has a correlation to the preceding signal, and the original code is detected by inferring the bit sequence having the maximum likelihood, in other words, the highest probability, on the basis of this correlativity.
    10 recording medium    20 image sensor    21 second lens    25 encoder    26 decoder    32 controller    16 objective lens    HM beam splitter    LD light source    SH shutter    BX beam expander    SLM spatial light modulator